This invention relates to a coaxial engine starter in which an armature rotary shaft of a d.c. motor, a rod of a solenoid switch and a starter output rotary shaft are disposed on a common axis.
FIG. 1 illustrates in a sectional front view one example of a conventional coaxial engine starter 20 which comprises a d.c. motor 1 having a hollow armature rotary shaft 2 within which an output rotary shaft 3 having a pinion 4 for engagement with an engine ring gear (not shown) is coaxially slidably inserted. Behind the d.c. motor 1, a solenoid switch 5 is disposed. The solenoid switch 5 comprises a magnetically movable iron core or plunger 6 magnetically driven by a solenoid coil 5a. The plunger 6 includes a plunger rod 6a coaxially slidable relative to the armature rotary shaft 2 for operatively engaging the output rotary shaft 3 within the cylindrical space 2a or a bore of the hollow armature rotary shaft 2 through a push rod 8 biased by a spring 7. The plunger rod 6a has mounted thereon a movable contact 10. The d.c. motor 1 comprises a substantially cup-shaped rear bracket 9 which is a part of the motor housing for supporting therein electrical brushes (not shown) of the d.c. motor 1. A pair of stationary contacts 13 for being engaged by the movable contact 10 are mounted to the rear bracket 9. A bushing 14 is disposed around the outer circumferential surface of the plunger 6 along which the plunger 6 is moved. A resin mold portion 15 and a cover 16 are also disposed.
The rear bracket 9 which is the motor housing has a hollow cylindrical portion 17 defining therein a central bore within which a sleeve bearing 18 is supported. The sleeve bearing 18 rotatably supports the outer surface of the rear end of the armature rotary shaft 2. It is seen that the front end of the plunger rod 6a is inserted into the rear end of the armature rotary shaft 2 and defines a substantially annular clearance between the cylindrical portion 17 and the plunger rod 6a as well as between the inner surface of the armature rotary shaft 2 and the plunger rod 6a.
When an unillustrated starter switch of a vehicle is turned on, the solenoid switch 5 is energized to move the plunger 6 forward, and this forward drive force is transmitted to the output rotary shaft 3 through the tubular plunger rod 6a and the intermediate push rod 8. Also, at this time, the movable contacts 10 abut against the stationary contacts 13 to connect the d.c. motor 1 to the power source (not shown), so that the rotational drive force of the armature rotary shaft 2 is transmitted to the output rotary shaft 3 through an unillustrated drive force transmission mechanism. This rotation is transmitted to the pinion 4 which is in engagement with the engine ring gear (not shown) due to the forwardly moved output rotary shaft 3, whereby the engine (not shown) is started. Further, when the power source is disconnected from the solenoid switch 5 after the engine has been started, the output rotary shaft 3 is returned to its home position by a return spring (not shown), thereby to release the engagement of the pinion 4 and the engine ring gear (not shown).
As has been described, the conventional coaxial engine starter has the hollow armature rotary shaft 2 that has the bore 2a of a small inner diameter because of the dimensional limitation imposed upon the bearing 18 supporting the rear end of the armature rotary shaft 2, so that there is only a small clearance between the plunger rod 6a and the inner surface of the armature rotary shaft 2. Therefore, when the plunger rod 6a tilts due to a clearance between the plunger and the bushing or the like, the front end of the plunger rod 6a is brought into contact with the inner surface of the rear end of the armature rotary shaft 2, generating disadvantageous wear accompanied by noise and undesirable jerky movement of the plunger rod 6a.
Also, wear powder generated by the sliding movement between various movable parts such as the movable contact 10, the stationary contacts 13 and the plunger 6 may enter into the annular clearance between the sleeve bearing 18 and the outer surface of the rear end of the armature rotary shaft 2 as well as between the inner surface of the armature rotary shaft 2 and the outer surface of the plunger rod 6a, whereby the smooth movement of the plunger rod 6a is impeded. Further, the lubricating greace at the sliding or rotating surfaces between the movable components may intrude into the contact portion of the movable and stationary contacts 10 and 13 of the solenoid switch 5, preventing a proper electrical connection between the movable and the stationary contacts 10 and 13.